WEDNESDAY, Feb. 6 (HealthDay News) -- Pregnant women exposed to particulate air pollution -- commonly known as smog -- have a significantly greater risk of having a baby with a low birth weight, according to a large new international study.

Specifically, particulate air pollution refers to tiny particles emitted by vehicles, coal power plants and other sources. Low birth weight (less than 5.5 pounds) is associated with increased likelihood of complications and death after birth, as well as chronic health problems later in life.

Researchers analyzed data from more than 3 million births in North America, South America, Europe, Asia and Australia. They found that the greater the amount of particulate pollution, the higher the rate of babies with low birth weight.

The study was published Feb. 6 in the journal Environmental Health Perspectives. Although it shows an association between air pollution and low birth weight, it doesn't prove a cause-and-effect relationship.

"What's significant is that these are air-pollution levels to which practically everyone in the world is commonly exposed," study co-principal investigator Tracey Woodruff, a professor of obstetrics and gynecology and reproductive sciences at the University of California, San Francisco, said in a university news release. "These microscopic particles, which are smaller than the width of a human hair, are in the air that we all breathe."

Woodruff noted that nations with tighter regulations on particulate air pollution have lower levels of these pollutants.

"In the United States, we have shown over the last several decades that the benefits to health and well-being from reducing air pollution are far greater than the costs," Woodruff said. "This is a lesson that all nations can learn."

Study co-author Mark Nieuwenhuijsen, of the Centre for Research in Environmental Epidemiology in Spain, said in the news release: "This study comes at the right time to bring the issue to the attention of policy makers."

Nieuwenhuijsen noted the recent exceedingly high levels of particulate air pollution in Beijing. "From the perspective of world health, levels like this are obviously completely unsustainable," he said.